The bacterium Listeria monocytogenes (Lm) is one of the most deadly foodborne pathogens, and is particularly dangerous in the immune compromised. How Listeria causes disease and why there is increased susceptibility in some populations remains poorly understood. During infection, Listeria and other microbes can initiate host responses that suppress, rather than contribute to, protective immunity. Identifying how microbes inhibit host immunity is integral to our understanding of host-pathogen interactions and the design of therapies to treat infection. Preliminary data suggest that Lm induces a natural killer (NK) cell-dependent, systemic IL-10 response that limits bacterial clearance. The goals of this proposal are to investigate the induction of NK cell IL-10 production and determine how NK cell-dependent IL-10 limits protective immunity to Lm. First, the stimulation of NK cell-dependent IL-10 in the context of Lm infection will be explored. The effect of IL-18, which contributes to early NK cell IFN-? production, on NK cell-dependent IL-10 expression will be determined using a co-culture system (with purified NK cells and IL-10-/- dendritic cells) in conjunction with in vio infections in mice that are unresponsive to IL-18. It is also possible that early NK cell IFN-? affects subsequent IL-10 production. The potential for feedback regulation between NK cell-dependent IFN-? and IL-10 will be investigated by transferring NK cells from mice that are unresponsive to either IFN-? or IL-10 into Lm-infected hosts. In the second half of the proposed work, the impact of NK cell-dependent IL-10 on protective immunity will be investigated. Mice with immune cell subsets that are unresponsive to IL-10 will be infected with Lm to determine the cell types required for IL-10-mediated susceptibility. Finally, the impact of NK cell-dependent IL-10 on immune cell protective activity will be evaluated in vivo by measuring CD8+ T cell cytotoxicity during Lm infection in wild-type versus IL-10-/- mice. Collectively, these experiments will explore the requirements for Lm-induced NK cell IL-10 production and establish how this response impacts host protective immunity. This work will expand our knowledge about how microbes can suppress host immunity during infection, which has broad implications for a number of diseases in which the immune response is inappropriately activated.